The problem with smart people is that they like to be right and sometimes will defend ideas to the death rather than admit they’re wrong. This is bad. Worse, if they got away with it when they were young (say, because they were smarter than their parents, their friends, and their parent’s friends) they’ve probably built an ego around being right, and will therefore defend their perfect record of invented righteousness to the death. Smart people often fall into the trap of preferring to be right even if it’s based in delusion, or results in them, or their loved ones, becoming miserable. (Somewhere in your town there is a row of graves at the cemetery, called smartypants lane, filled with people who were buried at poorly attended funerals, whose headstones say “Well, at least I was right.”)
Until they come face to face with someone who is tenacious enough to dissect their logic, and resilient enough to endure the thinly veiled intellectual abuse they dish out during debate (e.g. “You don’t really think that do you?” or “Well if you knew the <insert obscure reference here> rule/law/corollary you wouldn’t say such things”), they’re never forced to question their ability to defend bad ideas. Opportunities for this are rare: a new boss, a new co-worker, a new spouse. But if their obsessiveness about being right is strong enough, they’ll reject those people out of hand before they question their own biases and self-manipulations. It can be easier for smart people who have a habit of defending bad ideas to change jobs, spouses, or cities rather than honestly examine what is at the core of their psyche (and often, their misery).